The present invention relates to a drive circuit for a liquid crystal display apparatus of active matrix type, or more in particular to a liquid crystal display apparatus having a drive circuit formed on the same substrate as an active matrix substrate.
The liquid crystal display apparatus of active matrix type comprises a display unit formed with a transistor at each intersection of a plurality of signal lines and scanning lines arranged orthogonally to each other and a drive circuit unit for controlling the voltages of the signal lines and the scanning lines. The transistors used for the display unit include an amorphous silicon (a-Si) thin-film transistor (TFT), a poly-silicon (p-Si) thin film transistor, and a single-crystal silicon MOS (Metal-Oxide Semiconductor) transistor. The a-Si TFT is formed on a glass substrate, and as a drive circuit thereof, a single-crystal silicon integrated circuit is mounted externally. The p-Si TFT is either a high-temperature p-Si TFT formed on a quartz substrate or a low-temperature p-Si TFT formed on a glass substrate. The drive circuit for the liquid crystal display apparatus using the p-Si TFT is formed on the same substrate as the display unit. The amorphous silicon TFT or the low-temperature p-Si TFT formed on the glass substrate can realize a large size screen, while a quartz substrate or a single-crystal silicon substrate is limited to a screen of small or medium size.
The configuration and the operation of the liquid crystal display apparatus of active matrix type will be described in more detail.
Each of the transistors of the display unit has a gate connected to a scanning line, a drain to a signal line and a source to a display electrode. Another substrate formed with a transparent electrode on one surface thereof is arranged in opposed relation to the display electrode, and the liquid crystal is held between the display electrode and the opposed substrate. Normally, the display electrode is connected with a holding capacitor, and therefore the holding capacitor and a liquid crystal capacitor are connected in parallel to the source electrode. When the gate electrode enters a select mode, the transistor turns on, and writes the video signal from the signal line into the liquid crystal capacitor and the holding capacitor. When the gate electrode turns to a non-select mode, on the other hand, the transistor comes to assume a high impedance, and holds the video signal written in the liquid crystal.
The drive circuit unit includes a scanning circuit for controlling the voltage on the scanning line and a signal circuit for controlling the voltage on the signal line. The scanning circuit applies a scan pulse once per frame time to each scanning line. Normally, this pulse timing is differentiated sequentially downward of the panel. One frame time of {fraction (1/60)} second is often used. A panel having a typical pixel array of 1024xc3x97768 dots is scanned 768 times per frame time, and therefore the time width of the scan pulse is about 20 xcexcs. This scanning circuit is normally configured with a shift register which has an operating speed of about 50 kHz.
On the other hand, the signal circuit supplies each signal line with a liquid crystal drive voltage corresponding to the pixels for one line applied with the scan pulse. In the selected pixels applied with the scan pulse, the voltage of the gate electrode of the transistor connected to the scanning line increases and turns on the transistor. In the process, the liquid crystal drive voltage is applied to the liquid crystal through the drain and the source of the transistor from the signal line, thereby charging the pixel capacitor including the liquid crystal capacitor and the holding capacitor. By repeating this process of operation, a voltage corresponding to the repetitive video signal is applied to the pixel capacitors over the whole surface of the panel for each frame time.
In the case of analog system, the signal circuit for driving the signal line is configured with a shift register and a sample/hold circuit. The shift register generates a timing for the sample/hold circuit corresponding to each pixel. In the sample/hold circuit, the video signal corresponding to each pixel is sampled at this timing, and the liquid crystal drive voltage is supplied to each signal line. This driving method can be realized with a simple circuit configuration including a shift register for generating a timing and a sample/hold circuit for sampling the video signal. This driving method, therefore, is used primarily for a liquid crystal display panel integrated with a drive circuit.
In the aforementioned pixel configuration, the shift register of the signal circuit generates 1024 timing pulses with a time width of the scan pulse of the scanning circuit. As a result, the time interval of the timings of the shift register is not more than 20 ns, and therefore this shift register requires an operating speed of not less than 50 MHz. The sample/hold circuit is required to sample the video signal with this short timing. In the liquid crystal display apparatus integrated with the drive circuit, the sampling time is lengthened by dividing the video signal into a plurality of portions and inputting them in parallel. For this purpose, a signal conversion circuit is required in which a high-speed video signal is split into a plurality of video signals by sampling and the signals thus split are amplified and converted into an alternating current.
In the digital system, on the other hand, the signal circuit for driving the signal lines includes a shift register, a two-stage latch circuit, and a digital-to-analog converter (hereinafter referred to as D/A converter). The video signals sequentially input in digital form are stored in the latch circuit corresponding to each signal line by the shift register and the two-stage latch circuit. The D/A converter converts this data into an analog voltage and thus supplies the liquid crystal drive voltage to each signal line.
The number of bits for the latch circuit and the D/A converter of this system is determined by the gradation to be displayed, and is 8 in the case where 256 tones of each color is required for full-color display. In the pixel configuration described above, a latch circuit of 16384 bits (8 bitsxc3x972xc3x971024) and 1024 8-bit D/A converters are required. In the D/A converter for each signal line, the reference voltage is selected by switch in order to reduce variations. In this digital system, the video signal is a digital signal, and therefore the S/N (signal-to-noise) ratio can be prevented from deteriorating at the time of signal transmission.
In both the analog system and the digital system described above, the signal line is required to be driven with an accurate voltage for displaying a high-quality image. The signal line is a capacitive load, the capacitance of which is determined by the capacitance between the drain and gate of the transistor making up the display unit, the crossing capacitance between the signal line and the scanning line and the capacitance between the signal line and the transparent electrode of the opposed substrate. The capacitance of the capacitive load of the signal line, therefore, increases with the size of the display unit and the number of pixels making it up. In the a-Si TFT capable of realizing a large-sized display apparatus, therefore, a buffer amplifier for driving the capacitive load of the signal line is used as an external drive circuit configured with an integrated circuit. With this drive technique using the buffer amplifier, the offset voltage is required to be reduced with the capacitance drivability. This can be achieved by either a method using a differential amplifier circuit or a method using a source-follower amplifier circuit.
A method using a differential amplifier circuit is described, for example, in JP-A-5-297830. This method is an application to a signal line drive circuit of analog type and performs the function of a sample/hold circuit at the same time. This signal line drive circuit includes a differential amplifier circuit, and a capacitor and a switch for holding the voltage and offset correction. The offset voltage of the differential amplifier circuit is corrected by holding the offset voltage in a capacitor.
A method using a source-follower amplifier circuit, on the other hand, is described in U.S. Pat. No. 5,266,936, for example. This is used for a digital signal line drive circuit, and is arranged at the output of a D/A converter for each signal line. This circuit includes a source-follower circuit, a voltage holding circuit, a comparator circuit and a ramp voltage circuit. The source-follower circuit is driven by the ramp voltage circuit through the voltage holding circuit, and the voltage holding circuit is rendered in a hold condition by assuring that the output voltage of the source-follower circuit is coincident with the input voltage of the comparator circuit.
A drive circuit for an a-Si TFT which can realize a large-sized display apparatus is a single-crystal Si integrated circuit provided externally. Under the circumstance, this drive circuit is provided for each set of about 300 signal lines in order to keep the occupied area of the integrated circuit in a practical range. In similar fashion, for a low-temperature p-Si TFT capable of realizing a large-sized display apparatus, the drive circuits of all the signal lines for display are required to be formed on the same substrate. The number of signal lines is 1024 in the aforementioned case. Three times as many signal lines, i.e. 3072 signal lines are required for color display.
As described above, in the liquid crystal display apparatus integrated with a drive circuit, the signal lines about ten times as many as those driven by the conventional single-crystal Si integrated circuit are required. In the prior art, therefore, the area occupied by circuits on the substrate is increased for the signal line drive circuit. This makes it difficult to use the drive circuit for the liquid crystal drive unit integrated with a drive circuit.
An object of the present invention is to provide a liquid crystal display apparatus capable of reducing the area occupied by the drive circuit in the circuits of the liquid crystal display apparatus integrated with the drive circuit.
Another object of the invention is to provide a large-sized liquid crystal display apparatus integrated with a drive circuit.
The technical measures for achieving the objects described above will be explained below.
According to a first aspect of the invention, there is provided a drive circuit for a liquid crystal display panel comprising sample/hold circuits sampling an input voltage at a predetermined timing, comparator circuits comparing the output of the sample/hold circuit with the voltage on a signal line, switches controlling the output voltage of the signal line, voltage supply means for supplying a voltage to the switch, and control circuits controlling the switch by the output of the comparator circuits.
According to a second aspect of the invention, there is provided a liquid crystal display panel drive circuit adapted to assume a positive status in which the input voltage changes in the same polarity as the video signal and a negative status in which the input voltage changes in the opposite polarity to the video signal, the drive circuit comprising a first drive circuit operated by a positive input voltage and a second drive circuit operated by a negative input voltage.
According to a third aspect of the invention, there is provided a liquid crystal display panel drive circuit of the second aspect, in which the first drive circuit includes a N-type transistor as a switch, and the second drive circuit includes a P-type transistor as a switch.
According to a fourth aspect of the invention, there is provided a liquid crystal display panel drive circuit of the second aspect, in which the operation of the drive circuit is divided into a sampling period when the input voltage is sampled and an output drive period when the voltage on the signal line is driven, the first drive circuit having a sampling period and an output drive period different from the second drive circuit.
According to a fifth aspect of the invention, there is provided a liquid crystal display panel drive circuit of the first or second aspect, in which a logic circuit of inverter type is used as the sample/hold circuit and the comparator circuit.
According to a sixth aspect of the invention, there is provided a liquid crystal display panel drive circuit of the second aspect, in which a positive ramp voltage is used for the voltage supply means of the first drive circuit, and a negative ramp voltage is used for the voltage supply means of the second drive circuit.
According to a seventh aspect of the invention, there is provided a liquid crystal display panel drive circuit of the second aspect, in which a positive fixed power supply is used as the voltage supply means for the first drive circuit, a negative fixed power supply is used as the voltage supply means for the second drive circuit, and the switch is adapted to control the current of the constant current source connected to the fixed power supply.
According to an eighth aspect of the invention, there is provided a liquid crystal display apparatus comprising:
a first substrate formed with a switching element at each intersection between the scanning lines and the signal lines, a scanning circuit for controlling the voltage of the scanning line, and a signal circuit for controlling the voltage on the signal line;
a second substrate having a transparent electrode formed on one side thereof;
a liquid crystal held between the first substrate and the second substrate; and
a set of drive circuits each having a signal circuit including a sample/hold circuit for sampling the input voltage constituting a video signal at a predetermined timing, a comparator circuit for comparing the output of the sample/hold circuit with the signal line voltage, a switch for controlling the output voltage of the signal line, a voltage supply means for supplying a voltage to the switch, and a control circuit for controlling the switch by the output of the comparator circuit.
According to a ninth aspect of the invention, there is provided a liquid crystal display apparatus of the eighth aspect, wherein the drive circuit includes a first drive circuit turned on by a positive input voltage and a second drive circuit turned on by a negative input voltage.
According to a tenth aspect of the invention, there is provided a liquid crystal display apparatus comprising:
a first substrate on which there are formed at least a switching element at intersections between the scanning lines and the signal lines, at least a scanning circuit for controlling the voltage on the signal line, and a signal circuit for controlling the voltage on the signal line;
a second substrate having a transparent electrode formed on one side thereof;
a liquid crystal held between the first substrate and the second substrate; and
a signal circuit including drive circuits, each drive circuit comprising a D/A converter for converting a digital video signal input thereto into an analog voltage, at least a sample/hold circuit for sampling the analog voltage at a predetermined timing, at least a comparator circuit for comparing the output of the sample/hold circuit with the voltage on the signal line, at least a switch for controlling the output voltage of the signal line, at least a voltage supply means for supplying a voltage to the switch, and at least a control circuit for controlling the switch by the output of the comparator circuit.
According to an eleventh aspect of the invention, there is provided a liquid crystal display apparatus comprising:
a first substrate on which there are formed a switching element at each intersection between the scanning lines and the signal lines, a scanning circuit for controlling the voltage on the scanning line, and a signal circuit for controlling the voltage on the signal line;
a second substrate having a transparent electrode formed on one side thereof;
a liquid crystal held between the first substrate and the second substrate;
a first D/A converter for converting a digital video signal input thereto into a positive analog voltage;
a second D/A converter for converting a digital video signal input thereto into a negative analog voltage;
a first drive circuit supplied with a positive analog voltage for sampling the positive analog voltage at a predetermined timing and driving the signal line; and
a second drive circuit supplied with a negative analog voltage for sampling the negative analog signal at a predetermined timing and driving the signal line.
According to a twelfth aspect of the invention, there is provided a liquid crystal display apparatus of the eleventh aspect, wherein:
the first drive circuit includes a first sample/hold circuit for sampling a positive analog voltage, a first comparator circuit for comparing the output of the first sample/hold circuit with the voltage of the signal line, a first switch for controlling the output voltage of the signal line, first voltage supply means for supplying a voltage to the first switch, and a first control circuit for controlling the first switch by the output of the first comparator circuit; and
the second drive circuit includes a second sample/hold circuit for sampling a negative analog voltage, a second comparator circuit for comparing the output of the second sample/hold circuit with the voltage of the signal line, a second switch for controlling the output voltage of the signal line, second voltage supply means for supplying a voltage to the second switch, and a second control circuit for controlling the second switch by the output of the second comparator circuit.
According to a thirteenth aspect of the invention, there is provided a liquid crystal display apparatus, comprising:
a first substrate on which there are formed at least a switching element at each intersection between the scanning lines and the signal lines, at least a scanning circuit for controlling the voltage on the scanning line, and a signal circuit for controlling the voltage on the signal line;
a second substrate having a transparent electrode formed on one side thereof;
a liquid crystal held between the first substrate and the second substrate;
first and second latch circuits for holding continuous digital video signals;
switching means for switching the outputs of the first and second latch circuits to each other;
a first D/A converter connected to one of the outputs of the switching means for converting a digital video signal into a positive analog voltage;
a second D/A converter connected to the other output of the switching means for converting a digital video signal into a negative analog voltage;
first and second analog signal bus lines connected to the first and second D/A converters, respectively; and
first and second output means connected to the first and second bus lines, respectively, for driving the first and second signal lines, respectively;
wherein the first and second output means each include a first drive circuit connected to the first bus line for controlling the positive voltage and a second drive circuit connected to the second bus line for controlling the negative voltage;
wherein a first operation is performed in such a manner that the output of the first latch circuit is applied through the first D/A converter to the first drive circuit constituting the first output means thereby to drive the first signal line while the output of the second latch circuit is applied through the second D/A converter to the second drive circuit constituting the second output means thereby to drive the second signal line, and a second operation is performed in such a manner that the output of the first latch circuit is applied through the second D/A converter to the second drive circuit constituting the first output means to drive the first signal line while the output of the second latch circuit is applied through the first D/A converter to the first drive circuit constituting the second output means thereby to drive the second signal line.
Other objects, features and advantages of the present invention will become apparent from the following description of the embodiments of the invention taken in conjunction with the accompanying drawings.